In a plasma welding or process, where the resulting weld or cut quality is of a prime importance, it has been known to be necessary to perform the operation with a plasma torch while maintaining the distance between a nozzle forward end thereof and a workpiece, i.e. a standoff, constant. Thus, a plasma torch has typically been employed having a member that is mounted to a body portion thereof and designed to surround a nozzle forward end thereof, and such a plasma torch has been configured in a construction as disclosed in Japanese Examined Patent Publication No. Hei 3-9902 and Japanese Unexamined Patent Publication No. Hei 7-16752 in the prior art.
And, in a conventional plasma torch as disclosed in the former publication above, it can be noted that a spring member composed of a material having a heat resistant property is externally provided on the body of the plasma torch to form a coiled spring thereon whose leg end, extending downwards beyond the nozzle forward end, can be urged to engage with a workpiece so as to cut the workpiece. In another conventional plasma torch as disclosed in the latter publication above, it can be noted that the end of a retention cap mounted to the torch end extends downwards beyond the nozzle forward end and is urged to engage with a workpiece so as to spot weld the workpiece with another workpiece.
In these techniques in the prior art, however, it is recognized that a member as provided on a plasma torch and urged to engage with a workpiece must be subjected to a considerably elevated temperature that is brought by a heat of the plasma arc and a heat conducted from the workpiece. Thus, even if such a member contacting the workpiece is composed of a ceramic material to constitute a cap as have been used for a plasma torch and so forth, the contacting member is found to be disadvantageous on account of the productivity and the running cost because it tends to be broken or cracked.
On the other hand, in a case where the contacting member is composed especially of a metal, such as copper, having a high thermal conductivity, the member that is naturally of a non-insulating body allows a high frequency electric current that may be brought about when a plasma arc is started to leak therethrough into the workpiece, thus giving rise to a problem such as tending to develop an abnormal discharge or to make a plasma arc hard to develop.
In order to resolve the problem arising from leakage of such a high frequency current, it has been proposed as disclosed in Japanese Examined Utility Model Publication No. Hei 2-39657 that there be provided, between the end of a body portion of the plasma torch and the end of an insulating cap disposed coaxially to surround its nozzle portion with a spacing through which a shielding gas is allowed to pass, a protective packing that is composed of an insulating material and arranged to surround these two end portions so that the high frequency current may not leak through the gap between the torch body and the insulating cap into a region of the workpiece.
More specifically, a plasma torch as disclosed in the above mentioned utility model publication makes use of a packing interposed between the end of a torch body portion of the plasma torch and the end of an insulating cap disposed to coaxially surround a nozzle portion of the plasma torch with a spacing that is traversed by a shielding gas, where the packing is composed of an electrically insulating and heat resistant material such as to prevent a high frequency current from leaking from a region of the plasma torch into a region of the workpiece, and hence to prevent a development of any abnormal discharge.
With the use of a plasma torch of such a construction to carry out a welding or cutting operation for a workpiece while bringing a portion of the torch (here the insulating cap) in contact with the workpiece, it has nevertheless been found that the torch, even if its insulating cap is composed of a heat resistant material such as a ceramic, remains poor in the heat resistant property and hence its life is still extremely short as it is exposed to a plasma arc whose temperature is rising to as high as several ten thousand degrees.
In order to overcome the heat resistance problem, it has been proposed that the cap be composed of a metallic material that is good in thermal conduction and yet be cooled by water while the operation is being performed. Yet with such a measure, however, it has been found that the problem of a high frequency current leaking via the metallic cap into the region of a workpiece and hence the problems of development of an abnormal discharge and failure for an arc plasma to ignite may still be encountered.
At this point an explanation of a plasma arc process that has been employed with an assisting metallic cap in the prior art may be advisable.
A plasma arc with a plasma torch can be started with an arc current of low amperage, commonly referred to as "pilot arc", that is initially produced between an electrode and a torch nozzle of the plasma torch, the pilot arc being then allowed to reach a workpiece and thence to shift into a main arc that is produced between the electrode and the workpiece.
The main arc is called "plasma arc" because of its nature, and is characterized by an extremely elevated energy density and a high arc directivity.
An explanation of a mechanism whereby a plasma arc as mentioned above is allowed to ignite can be given with reference to FIG. 1 of the drawings attached hereto.
In order for a plasma arc to be ignited, it can thus be seen that a pilot arc needs to be developed between the electrode 1 and the torch nozzle 2. An arc turn-over switch 3 then remains closed, establishing a circuit in which current is allowed to flow from the turn-over switch 3 through a high frequency generator 5, the torch nozzle 2, a dielectric space 6 and the electrode 1 to a direct current (DC) power supply 4.
It should be noted, however, that to this end and thus to cause a pilot current to pass through this circuit the dielectric space 6 which lies between the electrode 1 and the torch nozzle 2 (and is filled up with a plasma gas) then must be broken down.
Accordingly, when a pilot arc is started, it will be required for a high frequency (HF) power with an extremely high voltage generated by a HF power supply (not shown) to be applied to the primary winding of a HF generator 5 to establish a resonant circuit which is formed by the HF generator 5, a capacitor 7a, the electrode 1, the torch nozzle 2 and the HF generator 5 that are connected in series, thereby allowing an elevated voltage HF current to be applied across the electrode 1 and the torch nozzle 2, thus permitting the dielectric space 6 to be broken down to establish a circuit for a pilot arc.
With the pilot arc then arriving at a workpiece 8, it follows that a main arc circuit will be established which is formed by the DC power supply 4, an ammeter 9, the workpiece 8, the electrode 1 and the DC power supply 4 that are connected in series. Here, the ammeter 9 is operative to detect a current that is indicative of the formation of this main arc circuit. Therefore, the switch 3 is turned off thereafter, thereby rendering the above mentioned pilot arc circuit in an open condition to extinguish any pilot arc and to allow it to shift into a main arc. It should also be noted that a second capacitor 7b is provided to isolate the HF current from the DC power supply 4 and the nozzle cap is indicated at 10a.
While FIG. 1 shows an example of circuit construction in which the HF generator 5 is connected to the torch nozzle 2, it should be noted that in another example of circuit construction the HF generator 5 may alternatively be connected to the electrode 1 as shown in FIG. 3. As will be appreciated, the mechanism for arc generation in the FIG. 1 example as discussed in connection therewith equally applies to the latter example as well.
Next, the mechanism for an arc ignition that is effected in the process of cutting or welding a workpiece while holding a standoff retention contact type cap 10b of the plasma torch in contact with a workpiece 8, may be explained with reference to FIG. 2 and in connection with the electrical circuit shown therein.
It has already been pointed that a standoff retention contact type cap 10b of the plasma torch of the type described, if composed of a ceramic or the like material on account of its required heat resistant property, leaves much to be desired as to the reliability, the running cost and so forth of a working operation. Note particularly that a ceramic is highly expensive and its utilization in such a manner may render the running cost of a process even prohibitive.
As a consequence, a need may arise that the standoff retention contact type cap 10b be composed of a metallic material having a high thermal conductivity. If so composed, the standoff retention contact type cap 10b will be placed at a same potential as the workpiece 8 and when the HF power supply is allowed to start its operation, it follows that the first circuit which is constituted by the electrode 1--the capacitor 7a--the HF generator 5--the torch coil 2--the dielectric space 6--the electrode 1 in series and the second circuit which is constituted by the standoff retention contact type cap 10b--the workpiece 8--the capacitor 7b--the HF generator 5--the torch nozzle 2--a second dielectric space 11--the standoff retention contact type cap 10b in series will be made structurally and functionally equivalent to each other as a whole.
For this reason, if a dielectric breakdown across the spacing 11 between the torch nozzle 2 and the standoff retention contact cap 10b happens to have been effected with an HF current prepared originally for effecting a dielectric breakdown across the space 6 between the electrode 1 and the torch nozzle 2, there will be no pilot arc produced or possibly ignited.
It may also be noted that even in a case where a dielectric breakdown is normally effected across the space between the electrode 1 and the torch nozzle 2 by using the former circuit above, if the conductor extending from the DC power 4 to the plasma torch is long in length, the impedance provided by the conductor (here, the self-inductance provided by the conductor with the power supply being an HF power supply) will be increased, thus causing a delay in charge passage in the torch nozzle 2, then possibly permitting a discharge to occur jumping from the torch nozzle 2 to the standoff retention contact type cap 10b to allow electric charges to transfer (a current to be passed) in the latter circuit mentioned above.
In the event that a main arc is then established in the state mentioned, it can be seen that not only the normal arc current from the electrode 1 to the workpiece 8 will be generated but also a current path from the electrode 1 through the torch nozzle 2 and the standoff retention contact type cap 10b to the workpiece 8 will be established. The phenomenon in which an electric current is diverted into a portion other than a location at which a normal arc may occur is called an "abnormal arc", which when produced would render a working current magnitude deficient, thus deteriorating a weld or cut quality of the workpiece 8 and also quite shortening the life of a consumable part of the plasma torch such as the electrode 1 or the torch nozzle 2.
Next, the mechanism of an arc ignition that is produced when performing a welding or cutting operation for a workpiece 8 while holding the standoff retention contact type cap 10b of a plasma torch in contact with the workpiece 8 with a circuit construction in which the HF generator 5 is connected to the electrode 1 as shown in FIG. 3, will be explained with reference to FIG. 4.
In the circuit construction of FIG. 4, it will only be the circuit constituted by the HF generator 5--the first capacitor 7a13 the torch nozzle 2--the first dielectric space 6--the electrode 1 which may develop when the HF power supply is allowed to start operating. It appears, accordingly, that a dielectric breakdown would be effected only across the first dielectric space and will not give rise to any inconvenience such as a development of abnormal discharge.
In the actuality, however, it has been found that an abnormal discharge does frequently develop, thus quite shortening the life of a consumable part here too, as in a previously described case where the HF generator 5 is connected to the torch nozzle 2.
It should be noted that even in a case where a dielectric breakdown of the first dielectric space 6 is normally effected, an abnormal discharge may frequently occur if the conductor extending from the DC power supply 4 to the plasma torch is greater in length, thus providing a greater resistance (greater value in the self-inductance of the conductor). This will cause electric charges to pass with a delay through the torch nozzle 2 and in turn a discharge to jump from the torch nozzle 2 to the standoff retention contact type cap 10b, thus forming a short-circuited path connecting the torch nozzle 2 through the workpiece 8 with the HF generator 5 and producing an abnormal discharge.
With the foregoing problems taken into account, it is accordingly an object of the present invention to provide a plasma torch for performing a working operation with respect to a workpiece by flushing a plasma arc drawn from an electrode, together with a working gas introduced from around the electrode, from a torch nozzle against the workpiece while holding a portion of the plasma torch substantially in contact therewith, in which that a portion of the plasma torch which makes a contact with the workpiece can be composed of a metallic material which is high in thermal conductivity and that a high frequency electric current is prevented from leaking from the plasma torch into a region of the workpiece and an arc may not fail to be ignited.